Cement kiln dust treatment system and method

ABSTRACT

A method for treating cement kiln dust is provided. The method includes the steps of receiving cement kiln dust (CKD) from a kiln; heating the collected CKD; forming a gas stream of vaporized metal and CKD by sufficient heating to separate at least one heavy metal from the collected CKD stream to create a cleaned CKD stream and a metal stream; providing a water soluble alkaline-earth metal polysulfide; combining the heavy metal stream with the water soluble alkaline-earth metal sulfide to create a combined stream; and removing at least a portion of one heavy metal from the combined stream.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/374,745, filed on Aug. 18, 2010, the contents ofwhich are incorporated herein by reference in their entirety.

FIELD

The present disclosure relates to heavy-metal pollution reduction in thecement kiln environment.

BACKGROUND

Cement kiln dust (CKD) is generated by cement kilns worldwide. Forexample, CKD is generated in cement kilns during the production ofcement clinker. Generally, CKD is a particulate mixture, including,amongst its constituents, partially calcined and unreacted raw feed,clinker dust and ash, enriched with alkali sulfates, halides, tracemetals and other volatiles.

CKD varies significantly dependent upon the specific plant process andraw materials. For reference purposes a CKD composition as reported bythe Bureau of Mines* includes:

Constituent % by Weight CaCO3 55.5% SiO2 13.6% CaO 8.1% K2SO4 5.9% CaSO45.2% Al2O3 4.5% Fe2O3 2.1% KCl 1.4% MgO 1.3% Na2SO4 1.3% KF 0.4% Others0.7%

* Typical Composition of Cement Kiln Dust (Haynes and Kramer, 1982)

CKD may be captured in a particulate collection system such as abaghouse or electrostatic precipitator. The captured CKD may then beutilized in a number of manners. For example, the captured CKD may beplaced back into the kiln process as a raw material; placed into thecement milling process as a process addition; sold as a cementitiousmaterial; and/or sent to a landfill.

The properties and composition of CKD can be significantly affected bythe design or operation of, or the materials used in, a cement kiln,with the result that constituent chemical and physical characteristicsof CKD must be evaluated on an individual plant basis. In general, thealkaline nature of the CKD makes it a good neutralizing agent whenadsorbing metals.

The CKD which is returned to the kiln on a routine basis undergoes aclinkering and/or calcination process, which causes compounds and traceelements to be volatilized. These materials are released back into thekiln gas stream. Over time, this often creates a cycle within the kilngas stream, building up concentrations of the elements and compounds inthe CKD which do not readily combine into clinker. As a result of thecement manufacturing process, materials such as mercury and other heavymetals may continue building in concentration. A portion of theseelements may be removed from the gas stream through the scrubbing effectof the raw mill, but only when it is in suitable operation. For example,pulverized limestone may absorb at least a portion of volatilized orother trace materials directly from the exhaust gas stream which is usedfor drying in a raw mill. However, when the raw mill is shut down, orwhen cement kiln or raw mill operations are otherwise unable to absorbsufficient amounts of elements of the exhaust gas stream, greaterquantities or concentrations of these elements may be released to theatmosphere through the cement kiln exhaust stack or absorbed by the CKD.Therefore, it is often desirable to treat the CKD before it is recycledback into the system.

SUMMARY

In one possible embodiment, a method for treating cement kiln dustincludes the steps of collecting cement kiln dust; heating the collectedcement kiln dust to separate at least one heavy metal from the collectedcement kiln dust to create a heavy metal stream; treating the heavymetal stream with a treating fluid; and removing at least a portion ofthe heavy metal from the heavy metal stream.

In one version, the step of heating the collected cement kiln dust mayinvolve heating to a temperature corresponding to the volatilizationpoint of the heavy metal to be treated. The step of heating thecollected cement kiln dust may include forming a modified kiln duststream. The cement kiln dust is “modified” or “clean” in the sense thatheavy metal such as mercury has been partially or completely removedfrom it. In other words, in modified cement kiln dust (mCKD), the amountof mercury has been reduced by the treatment described herein from thatgenerally present in cement kiln dust prior to treatment. The modifiedcement kiln dust stream may be recycled as a process addition, may bedisposed of, and/or may undergo further treatment.

In still further versions, the step of treating the heavy metal streamwith the treating fluid may include a treating fluid containing any ofthe following combinations: at least one alkaline-earth metalpolysulfide, an alkaline-earth metal polysulfide and water, analkaline-earth metal polysulfide and at least one surfactant, and/or analkaline-earth metal polysulfide and at least one catalyst agent. Othercombinations of the foregoing are also within the ambit of thisdisclosure, and other compositions may likewise be added to the treatingfluid, depending on the application.

In other possible implementations, the heavy metal stream is treatedwith a treating fluid so as to form a particulate containing the heavymetal; and the particulate is collected in a particulate collectionsystem.

In still another implementation, the step of treating the heavy metalstream with the treating fluid further includes spraying the treatingfluid into the heavy metal stream. Additionally, the step of collectingthe particulate containing the heavy metal may include passing theparticulate containing the heavy metal through at least one residuechamber. The method may further include the step of recycling theparticulate containing the heavy metal for use as a process addition, ormay involve further treatment or disposal.

A system for treating cement kiln dust according to this disclosureincludes a volatilization vessel; at least one heat source configured toheat a cement kiln dust stream within the volatilization vessel to forma gas stream containing at least one heavy metal; a treating chamber incommunication with the volatilization vessel; a treating fluidcontaining at least one alkaline-earth metal polysulfide; and at leastone nozzle within the treating chamber configured to spray the gasstream with the treating fluid to separate at least a portion of theheavy metal from the gas stream. The treating fluid may also includewater, and/or one or more catalyst agents.

In one implementation, the system may include a metering screw withinthe volatilization vessel configured to move the cement kiln dust streamthrough the volatilization vessel. A residue chamber or a particulatecollection system may be provided and configured to collect at least aportion of the heavy metal.

In one version, the source of the heat for the volatilization vessel maycome from transference of the heat of a cement kiln exhaust gas streamat the plant where the system is located. In addition, or alternately,the heat source may include an electrical heating element.

These and other aspects of the invention may be understood more readilyfrom the following description and the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the subject mattersought to be protected, an illustrative embodiment for treating cementkiln dust (CKD) is illustrated in the figure of the accompanying drawingwhich is meant to be exemplary and not limiting, in which likereferences are intended to refer to like or corresponding parts, and inwhich:

FIG. 1 schematically illustrates an embodiment of a system for treatingcement kiln dust (CKD).

DETAILED DESCRIPTION

While this invention is susceptible of embodiments in many differentforms, there is shown in the drawings and will herein be described indetail one or more detailed embodiments of systems, methods, andapparatuses for treating cement kiln dust (CKD), however, it is to beunderstood that the disclosed embodiments are merely exemplary of thesystems, methods, and apparatuses for treating CKD, which may beembodied in various forms. Therefore, specific functional detailsdisclosed herein are not to be interpreted as limiting, but merely as abasis for the claims and as a representative basis for teaching oneskilled in the art to variously employ the systems, methods, andapparatuses disclosed herein.

Heavy metals, such as mercury, may be managed through the systems,methods, and apparatuses of the present disclosure. These heavy metalsare derived primarily from raw materials which are chemically alteredduring a clinker process releasing these materials into a cement kilnexhaust gas stream containing CKD. These raw materials may includecalcium, silica, iron and alumina derived primarily from various formsof limestone, clay, shale, slags, sand, mill scale, iron-rich material(IRM), pumice, bauxite, recycled glass, ashes, and similar materials.For example, the cement kiln exhaust gases are typically passed from akiln through one or more processes, ducts, mills, cyclones, particulatecollection systems such as kiln bag houses, ESPs, or other particulatecollection systems. When the cement kiln exhaust gas stream is passedthrough a kiln baghouse, electrostatic precipitator (ESP), or otherparticulate collection system, all or a portion of the particulateswithin the cement kiln exhaust gas stream may be captured or collected.The collected particulate is typically a material referred to as cementkiln dust (CKD). The CKD can then be transferred to storage forcontrolled metering back into a cement kiln, cement grinding mill and/orused as a filler material within a concrete batch plant, asphalt plantor landfilled as non-leachable CKD.

A system and related processes for treating and/or reducing pollutionfrom CKD according to an illustrative embodiment is described below andshown schematically in FIG. 1. Since FIG. 1 is a schematic, it will beappreciated that the sizes, shapes, arrangements, and configurations ofthe system components and method steps may be varied and rest within thescope of the present disclosure. As illustrated, CKD 100 collected byone or more particulate collection systems is transferred to a storageunit, vessel, or container 102, including but not limited to a silo orother vessel, or the like. The CKD 100 may then be transferred to avolatilization vessel or zone 104, which may be in communication with orconnected to the container 102. In other illustrative embodiments, theCKD 100 may be transferred directly to the volatilization vessel 104,rather than being transferred to the storage unit 102.

The volatilization vessel 104 may take a variety of forms, including butnot limited to, ductwork, chambers, vessels, and the like. Asillustrated in FIG. 1, the CKD 100 is transferred to and/or through thevolatilization vessel 104 by a metering screw 106. However, it should beappreciated by one skilled in the art that the CKD 100 may betransferred to and/or through the volatilization vessel 104 byalternative means, including but not limited to, gravitational forces,conveyors, and/or other transferring devices or the like.

Generally, heat is added to the CKD 100 within the volatilization vessel104 to separate out and/or volatilize mercury and/or other heavy metalsto from the CKD 100. For example, mercury's boiling point is about356.58° C. or 673.844° F., after which point mercury exists as a gas,however other mercury compounds such as mercury chloride volatilize attemperatures as low as 302° C. In an illustrative embodiment, heat isapplied to the volatilization vessel 104 and/or the metering screw 106by one or more electric heating elements to heat the CKD 100 within thevolatilization vessel 104 to increase the temperature of the CKD 100.

In other illustrative embodiments, other heat sources may be used toincrease the temperature of the CKD 100 within the volatilization vessel104, including but not limited to the cement kiln exhaust gas stream,heat from the kiln, cooler, and/or other processes within the cementplant. It should be appreciated by one skilled in the art that the heatmay be applied to the CKD 100 external to or internal to thevolatilization vessel 104. Further, it should be appreciated by oneskilled in the art that a combination of heat sources may be used inseries or in parallel, for example the cement kiln exhaust gas streammay be used to partially heat the CKD 100, while one or more additionalheat sources are used to increase the temperature of the CKD 100 to thepoint of causing the heavy metals to release from the CKD 100.

Upon heating or increasing the temperature of the CKD 100 to thevolatilization point of mercury and/or other heavy metals, the mercuryand/or other heavy metals should vaporize into a heavy metal stream,i.e., a gas stream containing one or more heavy metals, generallycombined with air. As such, a heavy metal or gas stream 107 and amodified CKD stream 108 are formed by the heating step. Modified CKDstream 108 may be removed and/or recycled for use in other cement kilnprocesses via a duct 110. Duct 110 may be downstream of and connected toor in communication with the volatilization vessel 104. The gas, air, orheavy metal stream 107 containing vaporized mercury and possibly otherheavy metals may then be transferred to a treatment unit or injectionsystem through a duct 112, which may be upstream of and connected to orin communication with the volatilization vessel 104. In one possibleimplementation, the treatment unit includes but is not limited to,ductwork, chambers, cyclones, nozzles, and the like. As illustratedschematically in FIG. 1, the treatment unit includes a chamber ortreating chamber 114 and one or more nozzles 116 suitably positioned tocommunicate with the chamber 114. In this illustrative embodiment, thechamber 114 is downstream of and connected to or in communication withthe volatilization vessel 104 via duct 112. The nozzles 116 areconnected to or in communication with one or more vessels or containers118 for storing one or more fluids through one or more fluid connections120, such as pipes and/or hoses. The fluids are typically stored in thevessels 118 and transported through the fluid connections 120 to the gasstream in the chamber 114. The fluids can then be sprayed or injectedinto one or more ducts, chambers, or other process equipment carryingthe gas stream containing the vaporized mercury and/or other heavymetals to thereby treat and at least partially remove mercury and/orother heavy metals from the gas stream.

In an illustrative embodiment, the fluids or treating fluid is anaqueous spray solution. The treating fluid may be provided in a fullysoluble form enabling low cost application and retrofitting of existingfacilities. The treating fluid may comprise, include, consist, orconsist essentially of a reagent containing an alkaline-earth metalsulfide and/or polysulfide. The alkaline-earth metal sulfide and/orpolysulfide may have a pH of about 10 or more, and the treating fluidmay have a pH of about 7 to 10 dependent upon the concentration ofreagent in the treating fluid. In one embodiment, the reagent maycontain the alkaline-earth metal sulfide and/or polysulfide typically ata concentration of about 20% to 40% in water. In another embodiment, thereagent may contain the alkaline-earth metal sulfide and/or polysulfideat higher concentrations in water, or, alternately, may be in a powderor solid form having a substantially higher percentage, or consistingentirely, of the alkaline-earth metal sulfide and/or polysulfide. Thealkaline-earth metal sulfide/polysulfide may be added to another solid,powder, or liquid carrier to form the reagent.

In an illustrative embodiment, the reagent may comprise, include,consist, or consist essentially of an alkaline-earth metal polysulfidein water. The alkaline-earth metal polysulfide may be either a magnesiumor calcium polysulfide, and may be present in the reagent in an amountof about 25% to 35%, or about 25% to 30% in water. In anotherillustrative embodiment, the alkaline-earth metal polysulfide is amixture of magnesium polysulfides and calcium polysulfides, wherein thepolysulfides are present in the reagent in an amount of about 25% to35%, or about 25% to 30% in water.

In an illustrative embodiment, the treating fluid may comprise, include,consist, or consist essentially of the reagent and water. The treatingfluid may contain the reagent and water in a ratio of about 1:1 to 1:10,in a ratio of about 1:3 to 1:6, and more particularly in a ratio ofabout 1:4. When the reagent contains the alkaline-earth metal sulfideand/or polysulfide at a concentration of about 20% to 40% in water, theresulting treating fluid may contain the alkaline-earth metal sulfideand/or polysulfide and water in ratios of about 1:4 to about 1:54, in aratio of about 1:9 to 1:34, and more particularly in a ratio of about1:11 to 1:24. Thus, the alkaline-earth metal sulfide and/or polysulfidemay be present in the treating solution in an amount of about 1.8% to11%. However, it should be appreciated by one skilled in the art thatthe ratios of reagent to water and/or the alkaline-earth metal sulfideand/or polysulfide to water can vary outside of the ranges listed above.In many applications, the economic goal may be to use as little of thereagent and/or the alkaline-earth metal sulfide and/or polysulfide asoperationally possible. For example, the ratios used can vary dependentupon the amount of CKD, the concentration of mercury and/or other heavymetals in the gas stream, and other parameters of the type.

The reagent and water may be combined into the treating fluid prior toinjecting or spraying the treating fluid into the one or more ducts,chambers, or other process equipment carrying the gas stream containingthe vaporized mercury and/or other heavy metals. For example, thereagent and water may be combined well in advance of (i.e. one or morehours, days, weeks, months, etc. in advance) or just prior to (i.e. oneor more minutes prior to) injecting or spraying the treating fluid intothe one or more ducts, chambers, or other process equipment.

Alternatively, the reagent and water may each be separately sprayed orinjected into the one or more ducts, chambers, or other processequipment carrying the gas stream containing the vaporized mercuryand/or other heavy metals in a manner such that they intersect, combine,interact or coalesce in the one or more ducts, chambers, or otherprocess equipment to form a solution or composition in situ, formingdroplets of the solution or composition with the reagent reacting withthe metal(s) in the gas stream for removal.

The treating fluid may also contain one or more surfactants,dispersants, and/or hyperdispersants to assist in the removal ofmetal(s) from the gas stream containing the vaporized mercury and/orother heavy metals. In one embodiment, the surfactant, dispersant,and/or hyperdispersant are composed of one or more polyethyleneoxide-polyethylene block co-polymers and/or the phosphate estersthereof. The addition of the surfactant, dispersant, and/or hyperdispersant to the treating fluid may be optional. When the surfactant,dispersant, and/or hyper dispersant is included, the surfactant,dispersant, and/or hyper dispersant may be provided in an amountsufficient to assist in maintaining the reaction agent or reagent in thetreating fluid prior to reaction with the metal(s), for example in anamount of about 1% or less. According to the latter case, thesurfactant, dispersant, and/or hyper dispersant is a polyethyleneoxide-polyethylene block co-polymer and the phosphate esters thereof.

The treating fluid may also contain one or more catalyst agents toassist in or accelerate the removal of metal(s) from the gas streamcontaining the vaporized mercury and/or other heavy metals. The one ormore catalyst agents may accelerate the reaction of the reagent with themetal(s) in the gas stream, for example by releasing hydrogen sulfidefrom the reagent. In one embodiment, the catalyst agent is composed ofcalcium phosphate. However, it should be appreciated by one skilled inthe art that other catalyst agents may be used. The catalyst agent mayhave a pH of 7 or less. However, it should be appreciated by one skilledin the art that depending on the pH of the CKD and/or the gas streamcontaining the vaporized mercury and/or other metal(s), the addition ofthe catalyst agent to the treating fluid may be optional. When thecatalyst agent is included, the catalyst agent may be provided in anamount sufficient to assist in or accelerate the reaction.

In an illustrative embodiment, the treating fluid comprising, including,consisting, or consisting essentially of the reagent, water, the one ormore surfactants, dispersants, and/or hyper dispersants, and/or the oneor more catalyst agents may be combined into the treating fluid prior toinjecting or spraying the treating fluid into the one or more ducts,chambers, or other process equipment carrying the gas stream containingthe vaporized mercury and/or other heavy metals. For example, thereagent, water, the one or more surfactants, dispersants, and/or hyperdispersants, and/or the one or more catalyst agents may be combined wellin advance of (i.e. one or more hours, days, weeks, months, etc. inadvance) or just prior to (i.e. one or more minutes prior to) injectingor spraying the treating fluid into the one or more ducts, chambers, orother process equipment.

Referring back to FIG. 1, all or a portion of the volatilized mercuryand/or other heavy metals treated with the treating fluid in the chamber114 may precipitate or separate out of the gas stream as particulatesresulting in a clean gas or air stream. The precipitate or particulatesmay be filtered out, such as through a residue silo and/or a particulatefilter. In this regard, the collected mercury and/or other metal(s) canthen be further processed, recycled or otherwise disposed of properly.

In an illustrative embodiment, the particulates and clean gas stream maybe transferred through a duct 122 to a residue chamber, vessel, or silo124. As illustrated in FIG. 1, the duct 122 is downstream of andconnected to or in communication with the chamber 114, and upstream ofand connected to or in communication with the residue chamber 124. Theparticulates are transferred to the residue chamber 124 by a meteringscrew 126. However, it should be appreciated by one skilled in the artthat the particulates may be transferred to residue chamber 124 byalternative means, including but not limited to, gravitational forces,conveyors, and/or other transferring devices of the type. The residuechamber 124 generally collects all or a portion of the particulates fromthe treated gas stream. Further, the clean gas stream may be transferredthrough a duct 128, which may be upstream of and connected to or incommunication with the residue chamber 124, to one or more particulatecollection systems 130, which may be upstream of and connected to or incommunication with the duct 128, to further collect all or a portion ofthe remaining particulates in the clean gas stream. Any particulatescaptured in the particulate collection system(s) 130 and/or the residuechamber 124 may be recycled, for example by returning the capturedparticulates to the storage unit 102, or otherwise disposed of properly.The remaining clean gas stream may then be exhausted to the atmospherethrough the particulate collection system(s) 130.

In an illustrative embodiment, the system, method, and apparatusillustrated in FIG. 1 may be used to treat the CKD on a substantiallycontinual basis to remove and capture metals from all or a portion ofthe CKD, reducing or eliminating buildup and concentration of mercuryand/or other metals in the gas stream and particulate collectionsystem(s) during kiln operations. Alternatively, all or a portion of theCKD may be treated on a non-continuous basis to remove mercury and othermetals as their concentrations increase in the CKD. The non-continuousbasis may be tailored or in response to measurements of mercury andother metals at any number of points in the kiln or other components ofthe cement plant, such as before or after the particulate collectionsystem(s), or proximate to the exhaust stack. The non-continuous basismay also be triggered to run in response to any number of parameters,such as time, measurements of CKD constituents, and other parameters ofthe type. Each system, method, and apparatus may be tailored to eachcement kiln or plant based on the actual raw materials, costs, and anynumber of other operational, or functional parameters.

While the systems, methods, and apparatuses have been described andillustrated in connection with certain embodiments, it should beappreciated by one skilled in the art that the particulate collectionsystem(s) may be any of the variety of apparatus suitable for capturing,filtering, or otherwise collecting dust from the cement kiln's variousoperations including but not limited to a kiln baghouse, electrostaticprecipitator (ESP), or other particulate collection system. Given thevariety of cement kiln configurations possible, it should be appreciatedby one skilled in the art that the particulate collection system, aswell as the apparatus for treating the cement kiln dust, may beoperatively associated with the cement kiln, i.e., capable of receivingmaterial for treatment, at any number of suitable locations relative tothe components of the cement kiln. Further, depending on the volume ofresidual material generated, the portion which cannot be utilized as aprocess addition will have to be disposed of, but this is expected to bevery minor volume in the overall context.

The systems, methods, and processes disclosed herein have beenidentified, adapted to, and designed for the cement industry. In oneform, the systems, methods, and processes disclosed herein may provide alower capital cost, lower operating cost, and most importantly reducedmercury pollution levels. While the above description relates generallyto mercury capture, it should be appreciated that the systems, methods,processes, and technology disclosed herein may be modified to capturehexavalent chromium and a variety of other metals.

The matter set forth in the foregoing description and accompanyingdrawings is offered by way of illustration only and not as a limitation.While the systems, methods, and apparatuses have been described andillustrated in connection with certain embodiments, many variations andmodifications will be evident to those skilled in the art and may bemade without departing from the spirit and scope of the disclosure. Thesystems, methods, and apparatuses disclosed herein are thus not to belimited to the precise details of methodology or construction set forthabove as such variations and modification are intended to be includedwithin the scope of the disclosure.

1. A method for treating cement kiln dust, comprising: receiving cementkiln dust from a cement kiln; heating the cement kiln dust to separateat least one heavy metal from the cement kiln dust to create a heavymetal stream; treating the heavy metal stream with a treating fluid; andremoving at least a portion of at least one heavy metal from the gasstream.
 2. The method of claim 1, wherein heating the cement kiln dustincludes heating to a temperature at least as high as the temperaturecorresponding to the volatilization point of the heavy metal.
 3. Themethod of claim 1, wherein heating the cement kiln dust includes forminga modified cement kiln dust stream.
 4. The method of claim 3, furthercomprising recycling the modified cement kiln dust stream.
 5. The methodof claim 1, wherein treating the heavy metal stream with the treatingfluid further includes treating the heavy metal stream with a treatingfluid containing at least one alkaline-earth metal polysulfide.
 6. Themethod of claim 5, wherein heating the cement kiln dust includes heatingto a temperature at least as high as the volatilization point of mercuryto separate mercury from the cement kiln dust.
 7. The method of claim 6,wherein the treating step includes exposing the heavy metal stream tothe treating fluid to form a particulate containing the at least oneheavy metal; the method further comprising collecting the particulate inat least one particulate collection system.
 8. The method of claim 7,further comprising recycling the particulate containing the at least onemetal.
 9. The method of claim 7, wherein the collecting the particulatecontaining the at least one metal includes passing the particulatecontaining the at least one metal through at least one residue chambers.10. The method of claim 7, wherein treating the heavy stream furtherincludes spraying the treating fluid into the heavy metal stream. 11.The method of claim 1, wherein treating the heavy metal stream with thetreating fluid further includes treating the heavy metal stream with atreating fluid containing an alkaline-earth metal polysulfide and atleast one of the following: water, at least one surfactant, and at leastone catalyst agent.
 12. The method of claim 1, wherein receiving thecement kiln dust includes collecting cement kiln dust captured in acement kiln.
 13. A system for treating cement kiln dust, comprising: avolatilization vessel; at least one heat source configured to heat acement kiln dust stream within the volatilization vessel forming a gasstream containing at least one metal; a treating chamber incommunication with the volatilization vessel; a treating fluidcontaining at least one alkaline-earth metal polysulfide; and at leastone nozzle within the treating chamber configured to spray the gasstream with the treating fluid separating at least a portion of the atleast one metal from the gas stream.
 14. The system of claim 13, furthercomprising a metering screw within the volatilization vessel configuredto move the cement kiln dust stream through the volatilization vessel.15. The system of claim 13, wherein the collection system furtherincludes a residue chamber configured to collect at least a portion ofthe at least one metal.
 16. The system of claim 13, wherein thecollection system further includes a particulate collection systemconfigured to collect at least a portion of the at least one metal. 17.The system of claim 13, wherein the at least one heat source furtherincludes a cement kiln exhaust gas stream.
 18. The system of claim 13,wherein the at least one heat source further includes an electricalheating element.
 19. A method for removing mercury from cement kilndust, comprising: collecting cement kiln dust from a cement kiln;heating the collected cement kiln dust to a temperate at least as highas the volatilization point of mercury to form a stream of volatilizedmercury and a stream of modified cement kiln dust; separating the streamof modified cement kiln dust for further use; treating the stream ofvolatized mercury with a treating fluid containing at least onealkaline-earth metal polysulfide to form a particulate containing themercury; and collecting the particulate for subsequent handling.
 20. Themethod of claim 19, wherein the modified cement kiln dust is recycledinto a cement mill.